Field of the Disclosure
The present disclosure relates generally to optical communication networks and, more particularly, to nonlinear penalty estimation using spectral inversion in optical transport networks.
Description of the Related Art
Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical networks may also include various subsystems, such as amplifiers, dispersion compensators, multiplexer/demultiplexer filters, wavelength selective switches, spectral inverters, couplers, etc. configured to perform various operations within the network.
The distance that an optical signal can be transmitted with optical amplifiers for a given data rate depends on the impairments in the transmission system. Typically, the higher the data rate and the denser the wavelength spacing, the more susceptible the transmission system is to impairments. Impairments can include accumulated amplified spontaneous emission (ASE) noise, chromatic dispersion (CD), nonlinear optical effects (such as nonlinear phase noise), polarization mode dispersion, and polarization dependent loss. Digital signal processing (DSP) in coherent optical receivers may compensate for linear impairments such as CD, polarization mode dispersion and polarization dependent loss effectively. Intra-channel nonlinear impairment may also be compensated using digital back propagation in a coherent optical receiver with DSP, but such compensation may involve relatively extensive computational resources, which increases with optical signal bandwidth and is economically undesirable.
Nonlinear phase noise (NLPN) may be mitigated by mid-span spectral inversion when the optical signal is transmitted across multiple spans. Mid-span spectral inversion may be achieved optically (using optical phase conjugation based on an optical parametric process) or electronically (using an optical-electrical-optical (OEO) conversion). Accordingly, spectral inverters may change or maintain the wavelength after performing spectral inversion. The accumulated CD and NLPN of an optical signal may become reversed after spectral inversion is performed. Thus, to have optimal compensation of CD and NLPN, placement of spectral inverters has typically been limited to a central location (the mid-point) of a transmission link, such that the link is symmetric with respect to the spectral inversion. In real world systems, symmetric placement for spectral inverters (at the mid-point of the transmission link) may not be feasible or practical.